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Return vs. Supply Air Temperature

Does the temperature of the air entering the return directly affect the supply temperature on a heat pump system?

I'm asking because the finished room above our garage has the return (only 1) outside the room. Outside the room is a foyer and stairs, and there's only 1supply register in this space. It supposed to be conditioned space, but it's always very cold this time of year. I have to keep the room door closed all the time. The solid wood door is undercut for the return air. The VisionPro thermostat is located inside the room.

Just wondering if the return should have been in the fully conditioned room and what affect it's having on heating/cooling the room. Based on how cold the foyer/stairs is, I would like to have an insulated exterior door that is weatherstripped, but I know I can't block the return air exiting this room. Or can I?

Just a few questions to clear things up...
Does the finished room above your garage have a dedicated unit conditioning only this room and the foyer/stairs?
I assume you have another unit for downstairs?
Do the stairs go into the house or into the garage below?

return air temp affects supply temp, 50 degree return will make supply lower than 70 degree return. A heat pump can struggle to produce good temp when its cold outside. the south has been colder than normal, I have seen many heat pumps struggle to get more than 15 degree temp rise with below 30 degree ambient temperature.

The 1-1/2 ton heat pump/air handler are dedicated to this space (guest suite). The room is above a detached garage. The room door leads to small hallway which then leads to stairs to another small foyer/hallway like area at ground level. This little hallway has 2 doors. One door leads to the outside. The door opposite the exterior door leads to the garage. The air handler is located right outside the room in a cubby along with the return air.

Along the stairs are 2 fairly large windows, and at ground level there are 2 more windows. This side of the garage/guest room never receives direct sunlight.

For a variety of reasons there's no good way to add yet another door at the bottom of the stairs at ground level.

I'm very surprised a heat pump is more efficient with a lower return air temperature. I was thinking the guest room supply air temp might be low because of the colder than room temperature air return.

Thanks everyone for the info. I'm very happy to know the location of the return air OK. As to the cold stairs and hallway, I would expect that it would be cold given there's only one supply in the ceiling for the entire space. I'm not sure why that happened and I wish I would have caught it during the build.